An existing sensor for detecting the amount of PM (particulate matter) included in exhaust gas is an electrode-based PM sensor that includes electrodes arranged to be opposed to each other across an insulating layer and utilizes an electrical characteristic that the value of electric current flowing between the electrodes is varied according to the amount of PM depositing between the electrodes.
An existing method of diagnosing a failure (abnormality) of a particulate filter by utilizing an electrode-based PM sensor compares an output value of the PM sensor (value of electric current flowing between electrodes of the PM sensor) at the time when a predetermined time period has elapsed since termination of a process of removing PM depositing between the electrodes of the PM sensor (hereinafter referred to as “sensor recovery process”) with a predefined reference value, and diagnoses that the particulate filter has a failure when the output value of the PM sensor is higher than the predefined reference value.